This invention relates generally to a method and apparatus for determining a target. The invention has particular application in a navigation system which determines the range and/or angle to targets placed so as to define the desired path of an automated guided vehicle (AGV). The navigation system steers the vehicle to maintain it on a desired course, based on the detected range/angle information. The automated guided vehicle may, of course, serve any purpose. A machine for treating floor surfaces, such as a sweeper, scrubber or the like, is one example.
Devices for determining the range and angle of a target include the variety using a laser light source sweeping across a retroreflective target. Laskowski U.S. Pat. No. 4,788,441 and Stephens U.S. Pat. No. 4,647,784 show such devices incorporated in AGV's. Field U.S. Pat. Nos. 4,790,402 and 4,876,444 both assigned to the present assignee, illustrate the application of laser guidance to a floor treating machine. The disclosures of the Field patents are incorporated herein by reference.
It is desirable in a laser guidance system for AGV's to maximize the range at which the system can detect a target. This permits maximum flexibility in the placement of targets and reduces the total number of targets needed to define a given path. Maximum range is of particular importance in the floor treatment field. Floor treating machines must perform a task sometimes referred to as region filling. Region filling is where multiple passes of a machine are required to cover a floor area which is wide compared to the machine, e.g., a gymnasium. This task is simplified if the machine can make multiple passes guided by a minimum number of lines or rows of targets, ideally only a single line of targets. Each pass is made with a varying amount of offset from the line of targets. Greater sensor range increases the amount of offset, and consequently the amount of region filling, that can be obtained from a single line of targets.
Perhaps the most straightforward approach to increased range is to increase the power of the laser light source. However, safety considerations limit the power of any laser used to determine a target. The limits on power are dictated by present safety standards. Thus, the maximum range has already been reached with heretofore commercially available laser sources operating within the applicable power standard. Limits on range stem from the difficulty of distinguishing between valid signals generated by light reflected from targets and noise generated by ambient light. The farther away a target is, naturally, the weaker its reflected light signal is. The range limit is reached when a reflected light signal can no longer be reliably distinguished from noise.
Commercially available target determining systems have included gas-filled laser sources, such as a helium-neon laser. The gas-filled device does not permit effective encoding of its light output. However, there are now available semiconductor light sources, such as gallium arsenide laser diodes, which do permit additional encoding to be impressed on the emitted light. This encoding can be used to aid in distinguishing between weaker reflected light signals from distant targets and noise from ambient light. Specifically, a laser diode can be modulated and demodulated at a high frequency to exclude spurious signals, whereas this capability does not exist with a helium-neon laser. Stephens U.S. Pat. No. 4,647,784 suggests using a modulated laser diode in conjunction with a phase shift comparison. The present invention involves a different approach.